I'm a late comer to AJE, though I do own other Ageod titles. I'll probably have a few questions to ask in the coming days/weeks. But to start with I am unsure about how some of the regional decisions work.

Q. Merchants. This decision is only valid when played on a province you don't own - only enemy held regions are available to play this card. So who does the this benefit? Who gains the income that merchants generates?

I played the card on a region and then conquered it - but merchants was then immediately removed as no longer valid. I see no benefit to playing this decision if the income is only generated in enemy regions?

Merchants provide additional income. Presumably it represent Roman citizen entrepreneurs who travel to places not yet conquered to conduct trade with the locals. Once you have occupied an area it will begin to generate income which probably takes trade into account and cancels the Merchant regional decision for that area.

So it's a beneficial action to take at little cost and no risk even if the generated income isn't a lot.

A full list of regional decisions is available in the game manual, Appendix G starting on page 86.

I think I get it now. I found some cities on the edges of the map painted my enemy's colour due to their loyalties, but not actually at war with me. I played merchants there, followed by trade posts and I'm seeing an income from it detailed in my reports.